Discovered in 1987 by Paul Ching-Wu Chu's and Maw-Kuen Wu's teams, YBCO (Tc ≈ 93 K) was the first material to superconduct above the 77 K boiling point of liquid nitrogen.
Its orthorhombic defect-perovskite crystal structure conducts supercurrent along puckered CuO2 planes fed by Cu-O chains, with optimal superconductivity at oxygen deficiency x ≈ 0.07 (Tc up to 95 K) and insulating tetragonal behavior at x = 1.
Because grain-boundary misalignments above ~5° block supercurrent in brittle bulk ceramic, modern high-field fusion magnets (such as SPARC) deposit biaxially textured YBCO on buffered flexible metal tapes.